This invention relates to cross-linkable polymer compositions and to the cross-linked compositions prepared therefrom.
By reason of their desirable physical and chemical properties such as elasticity, pigment binding power and solvent resistance, synthetic polymer latexes are widely used as paints or coating materials for paper, carpets and the like. In such applications, the latexes, when applied, are aqueous dispersions of small (generally colloidal size), individual polymer particles which coalesce upon drying to form a continuous, adherent film. Upon aging, further coalescense improves the film's properties, e.g., increases toughness and solvent resistance.
To enhance the physical and chemical properties of the coatings prepared from the synthetic polymer latexes, it is often advantageous to cross-link (used interchangeably with the term cure) the polymers. Many polymers are prepared in latex form from monomers having sufficient functionality to polymerize and subsequently cross-link in the absence of a cross-linking agent. For example, a latex of a copolymer of styrene and butadiene contains residual carbon-to-carbon unsaturation which unsaturation provides cross-linking between polymer chains, thereby further enhancing the dried film properties. See, Treatise on Coatings, edited by R. R. Myers and J. S. Long, Vol. 1, published in 1968 by Marcel Dekker, Inc., New York, Chapter 1, "Styrene-Butadiene Latexes in Protective and Decorative Coatings," by F. A. Miller, pages 1-57. Alternatively, other reactive groups can provide the reactive sites for cross-linking. For example, U.S. Pat. No. 3,297,621 proposes preparing a curable latex composition by copolymerizing a monomer having carboxy groups such as methacrylic acid with a monomer having oxiranyl groups such as glycidyl methacrylate. Similarly, a curable latex composition of a polymer having a core of polyvinyl chloride and a shell derived from one or more of an .alpha.,.beta.-olefinically unsaturated carboxylic acid, an amine, a N-alkylol or N-alkoxyalkyl amide or an ester of acrylic or methacrylic acid is disclosed in U.S. Pat. No. 4,002,801. Other cross-linkable latexes are discussed in The Application of Synthetic Resin Emulsions, by H. Warson, published in 1972 by Ernest Benn Limited, London, Chapter IX, "Cross-Linking Industrial Finishes."
Unfortunately, such latex compositions cure relatively slow, often requiring a high temperature and/or long cure times. In many cases, curing temperatures of 150.degree. C. or above are required to sufficiently cross-link the latex film to provide desirable strength properties, hardness and resistance to chemical attack. Although these curing temperatures are economically undesirable, lower temperature cures often produce films of undesirable low strength properties and/or require extensive time to cure.
Alternatively, a cross-linking agent, i.e., a polyfunctional compound reactive with the polymers which reaction serves as a cross-linkage between the polymer molecules can be employed to cross-link the polymer particles. For example, U.S. Pat. No. 3,069,376 discloses that a dispersion of (1) a copolymer of an unsaturated carboxylic acid, a monohydric alcohol and an ester of an unsaturated carboxylic acid and (2) a ethoxyline resin cross-linking agent cures to a hard film. Similarly, a latex composition comprising a copolymer of an acrylic ester and an unsaturated azolide cures upon the addition of a polyfunctional alcohol or amine. See, U.S. Pat. Nos. 3,912,682; 3,955,041 and 3,956,216. Unfortunately, such latex compositions, which are relatively expensive, cure relatively slow; requiring lengthy cure times and/or high temperatures to achieve maximum film properties.
In view of the stated deficiencies of the prior art, it would be highly desirable to provide curable latex compositions which cure relatively quick at low temperatures sufficient to form strong, solvent-resistant films or coatings.